I have completed my course of Oracle 9i. Since my tutor induce us to use sqlplus rather than OEM console(GUI).
Could you pls tell y we should use sqlplus if GUI is well equipped and easy to use and understand.
what industry demands?

I believe your tutor didn't have any "industry" demands on mind. It is quite simple: if you learn how to use SQL*Plus and issue statements by yourself, you won't depend on GUI which - some day - might NOT be available. What will you do then?

Use SQL*Plus, it is available everywhere, it does give only what you asked, you know what it does.
GUI is pretty but in the end you don't what it does and what data it gives you.
If you only know how to work with SQL*Plus, you will easely know how to do it with a GUI.
If you only know how to work with a GUI, you will not know how to do it with SQL*Plus.

It's like learning math, you don't go into an arithmetic class for the first time with a calculator, you need to learn the basics first.
As you advance, the calculator becomes a tool to make things quicker, but if your calculator died, you would still be able to solve the problems.

A GUI is the same thing, a very valuable tool, but it should never be used to cover up lack of knowledge, but to help a developer be more productive.

I resisted GUI's for a long time, and succumbed about 3 years ago. I'm glad I did. I still go back to SQL*Plus on occasion just to keep my basic skills up to par.